Muscle fibre mitochondrial [Ca²⁺ ] dynamics during Ca²⁺ waves in RYR1 gain-of-function mouse

Aim: A fraction of the Ca²⁺ released from the sarcoplasmic reticulum (SR) enters mitochondria to transiently increase its [Ca²⁺ ] ([Ca²⁺ ]_mito ). This transient [Ca²⁺ ]_mito increase may be important in the resynthesis of ATP and other processes. The resynthesis of ATP in the mitochondria generates heat that can lead to hypermetabolic reactions in muscle with ryanodine receptor 1 (RyR1) variants during the cyclic releasing of SR Ca²⁺ in the presence of a RyR1 agonist. We aimed to analyse whether the mitochondria of RYR1 variant muscle handles Ca²⁺ differently from healthy muscle.

Methods: We used confocal microscopy to track mitochondrial and cytoplasmic Ca²⁺ with fluorescent dyes simultaneously during caffeine-induced Ca²⁺ waves in extensor digitorum longus muscle fibres from healthy mice and mice heterozygous (HET) for a malignant hyperthermia-causative RYR1 variant.

Results: Mitochondrial Ca²⁺ -transient peaks trailed the peak of cytoplasmic Ca²⁺ transients by many seconds with [Ca²⁺ ]_mito not increasing by more than 250 nM. A strong linear relationship between cytoplasmic Ca²⁺ and [Ca²⁺ ]_mito amplitudes was observed in HET RYR1 KI fibres but not wild type (WT).

Conclusion: Our results indicate that [Ca²⁺ ]_mito change within the nM range during SR Ca²⁺ release. HET fibre mitochondria are more sensitive to SR Ca²⁺ release flux than WT. This may indicate post-translation modification differences of the mitochondrial Ca²⁺ uniporter between the genotypes.